During the past decade, there has been an increasing interest in the evaluation of cognitive function in myasthenia gravis (MG), neuromuscular transmission disorder caused by acetylcholine receptor auto-antibodies. However, the results of previous studies on cognition and MG are inconsistent and controversial. This study aimed to evaluate cognition in patients with mild/moderate grades of MG. Methods: This study included 20 patients with MG with a mean age of 28.45 ± 8.89 years and duration of illness of 3.52 ± 1.15 years. Cognition was tested using a sensitive battery of psychometric testing (Mini-mental State Examination [MMSE], Stanford-Binet Intelligence Scale 4 th edition [SBIS] and Wechsler Memory Scale-Revised [WMS-R]) and by recording P300 component of event-related potentials (ERPs), a neurophysiological analog for cognitive function. Results: Compared with healthy subjects (n = 20), patients had lower total scores of cognitive testing (MMSE, SBIS and WMS-R) (P = 0.001), higher Beck Depression Inventory 2 nd edition scores (P = 0.0001) and prolonged latencies (P = 0.01) and reduced amplitudes (P = 0.001) of P300 component of ERPs. Correlations were identified between total scores of cognitive testing and age (r =-0.470, P = 0.010), duration of illness (r =-0.788, P = 0.001) and depression scores (r =-0.323, P = 0.045). Using linear regression analysis and after controlling for age and depression scores, a significant correlation was identified between total scores of cognitive testing and duration of illness (β =-0.305, P = 0.045). Conclusion: Patients with mild/moderate MG may have cognitive dysfunction. This is important to determine prognosis and managing patients.